The present invention relates to laser systems and more particularly to methods and devices for nonlinear frequency conversion of continuous-wave surface emitting diode lasers.
Diode lasers are compact and efficient sources of coherent light which are formed on semiconductor material using techniques developed for manufacturing integrated circuits. In a typical diode laser, all of the gain material and at least some of the reflective layers are formed in a single multi-layer semiconductor device.
Most diode lasers use a so-called xe2x80x9cedge-emitterxe2x80x9d geometry. For these lasers, the optical output is emitted from an aperture at one end of the semiconductor material. The emitter typically has a width on the order of 1 or 2 microns. The length of the emitter ranges from several microns for single-mode diode lasers to 10 millimeters or more for diode laser arrays.
Edge-emitting diode lasers have a number of drawbacks. Edge-emitting diode lasers have elliptical, divergent beams as a result of diffraction at the output aperture. Therefore, the beam must be optically corrected in order to collimate the beam over even a short distance. Moreover, the optical powers of edge-emitting diode lasers are often limited by the onset of catastrophic optical damage (COD) at the output aperture.
A significant amount of activity has recently been devoted to the development of xe2x80x9csurface-emittingxe2x80x9d diode lasers. For these devices, the optical output is emitted from the larger top or bottom surface of the semiconductor material. Consequently, the emitter aperture can be much larger than from an edge-emitter and much higher powers can be produced before reaching the onset of COD. In addition, a surface-emitting diode laser provides a circular beam which is less divergent than those produced by edge-emitting diode lasers.
In one common type of surface-emitting diode laser known as a vertical external cavity surface-emitting laser (VECSEL), multiple layers of distributed Bragg reflectors (DBRs) within the semiconductor material are used to form one end of a resonating cavity and a mirror, separate from the semiconductor material, forms the other end of the resonating cavity. To couple the larger surface-emitting gain region efficiently with a low order optical mode, the separate mirror is situated above the surface-emitter. Although the resonating cavity formed by the DBRs and the external mirror is commonly referred to as an xe2x80x9cexternalxe2x80x9d cavity, it will be referred to herein as a type of internal cavity.
The gain region of a surface-emitting diode laser typically has a thickness of only a few microns, which is only a few wavelengths of the emitted light. Consequently, the fundamental beam must pass through the gain material many times in order to develop a sufficiently high amplitude required by many applications. Therefore, both ends of the resonating cavity must be highly reflective for the desired output wavelength.
The output from most diode lasers, including VECSELs, is confined primarily to the near infrared portion of the spectrum. However, many applications require wavelengths in the visible or ultraviolet spectral region. The infrared output of diode lasers can be converted to the visible or ultraviolet by nonlinear optical frequency conversion such as optical frequency doubling.
The infrared outputs of VECSELs have been converted to the visible using intra-cavity frequency doubling. This approach involves inserting a nonlinear crystal directly into the resonator of the VECSEL, i.e., between the gain portion of the VECSEL and the outside mirror which forms one end of the VECSEL""s resonating cavity.
Intra-cavity frequency doubling with low-gain lasers such as VECSELs is problematic. Optical losses associated with the introduction of nonlinear optical crystals into the VECSEL resonator increase the threshold for the VECSEL, reducing efficiency. In addition, a portion of the infrared optical power circulating inside the VECSEL resonator is converted to the second harmonic in each direction. The second harmonic that is generated as the infrared beam travels from the gain region toward the external mirror can be out-coupled. However, the second harmonic generated in the nonlinear crystal as the infrared beam returns from the external mirror toward the semiconductor gain structure is also directed toward the semiconductor structure, where it is absorbed or de-phased relative to the infrared beam. Hence, this portion of the second harmonic light is lost and the nonlinear conversion efficiency is reduced by 50% or more.
The present invention provides devices and methods for efficiently converting the fundamental frequencies of surface-emitting diode lasers.
One aspect of the present invention provides a method of operating a solid state laser apparatus, including the steps of: pumping a surface-emitting diode laser to output a fundamental beam having a fundamental wavelength, the surface-emitting diode laser having a first resonating cavity; disposing a first nonlinear crystal in a second resonating cavity external to the first resonating cavity; directing the fundamental beam into the first nonlinear crystal; and tuning the first nonlinear crystal to generate a first output beam resulting from the interaction of the fundamental beam with the first nonlinear crystal, the first output beam having a first output wavelength different from the fundamental wavelength.
The first output beam may be directed into a second nonlinear crystal tuned to generate a second output beam having a wavelength different from the first output wavelength. The second nonlinear crystal may be disposed in a third resonating cavity external to the second resonating cavity. The second nonlinear crystal may be disposed in the second resonating cavity.
Some such methods include the steps of pumping an infrared laser to output an infrared beam, directing the infrared beam into the second nonlinear crystal and generating the second output beam by interaction of the infrared beam and the first output beam with the second nonlinear crystal.
According to another aspect of the present invention, a method of operating a solid state laser apparatus includes the steps of: pumping a surface-emitting diode laser to output a fundamental beam having a fundamental wavelength, the surface-emitting diode laser including a first resonating cavity; configuring a first surface and a second surface of a nonlinear crystal for total internal reflection to form portions of a second resonating cavity outside of the first resonating cavity; directing the fundamental beam into the nonlinear crystal; and tuning the nonlinear crystal to generate an output beam resulting from the interaction of the fundamental beam with the nonlinear crystal, the output beam having an output wavelength different from the fundamental wavelength.
According to some embodiments of the present invention, a laser apparatus includes: a surface-emitting diode laser including a first resonator; a pump for pumping the surface-emitting diode laser means to output a fundamental beam having a fundamental wavelength; a first nonlinear crystal for converting the fundamental beam to a first output beam having a first output wavelength different from the fundamental wavelength; a second resonator within which the first nonlinear crystal is disposed, the second resonator external to the first resonator; and an optical device for directing the fundamental beam into the first nonlinear crystal.
Some such embodiments include a second nonlinear crystal disposed within a third resonator external to the second resonator means for generating a second output beam having a wavelength different from the first output beam. Some embodiments include an infrared laser for outputting an infrared laser beam and an optical device for directing the infrared laser beam into the second frequency conversion means, wherein the second frequency conversion means generates the second output beam by interaction of the infrared beam and the first output beam with the second nonlinear crystal means.
Still other embodiments of the present invention provide a laser device including: a surface-emitting diode laser which outputs a fundamental beam having a fundamental wavelength, the surface-emitting diode laser comprising a first resonating cavity; a nonlinear crystal comprising a first surface and a second surface configured to form portions of a second resonating cavity disposed outside of the first resonating cavity; an optical device for directing the fundamental beam into the nonlinear crystal; and an adjustable mount for angle-tuning the nonlinear crystal to generate an output beam resulting from the interaction of the fundamental beam with the nonlinear crystal, the output beam having an output wavelength different from the fundamental wavelength.